Low molecular weight, or high melt index (MI), polyethylene (both homopolymer and copolymer) is widely used in bimodal resins, extrusion coating resins, and rotational and injection molding resins. In conventional polymerization processes, high MI polyethylene is produced by introducing hydrogen during the polymerization process. Unfortunately, the large quantity of hydrogen needed to produce high MI polymer suppresses the activity of the catalyst.
A few metallocene catalyst systems are reported that produce high MI polyethylene without the need for hydrogen. For example, some catalyst systems employing carbon-bridged cyclopentadienyl indenyl metallocenes have been shown to produce relatively lower molecular weight polyethylene when activated with methylaluminoxanes (MAO), but the activity of such catalyst systems is typically poor (Makromol. Chem. 1992, 193, 1643 and J Polym. Sci. Part A, 1994, 32, 2817).
Thus, there remains a need for a metallocene catalyst composition having a high activity, and a process for forming such a composition, that can be used to produce low molecular weight, high melt index polyethylene without the use of hydrogen.